In many industrial measurement applications there is a need for a sensor that can be used at high operating temperatures to measure the distance to either a stationary or passing object. A typical application is the measurement of clearance between the tip of a gas turbine engine blade and the surrounding casing. In this situation the operating temperature of the sensor can reach 1500° C. Other applications including molten metal and molten glass level measurement, for example, have similar operating temperature requirements.
U.S. Pat. No. 5,760,593 (BICC plc) describes a conventional sensor having a metal or metal-coated ceramic electrode that couples capacitively with the stationary or passing object. The electrode is connected directly to the centre conductor of a standard triaxial transmission cable and is surrounded by a metal shield and an outer housing. The metal shield and the outer housing are connected directly to the intermediate conductor and the outer conductor of the triaxial transmission cable respectively. Electrical insulation is provided between the electrode and the shield and also between the shield and the housing. The insulation can be in the form of machined ceramic spacers or deposited ceramic layers.
One problem with these conventional sensors is that they utilise an alternating combination of metal and ceramic materials. As the operating temperature of the sensor increases, the metal components tend to expand more than the ceramic components. This often results in stress fractures forming in the ceramic spacers or layers, which reduce their electrical performance and may even result in the disintegration or de-lamination of the ceramic components. Not only does this cause the sensor to fail electrically, but the disintegration or de-lamination of the ceramic components also allows the metal components to vibrate and this can result in the mechanical failure of the complete sensor assembly.
Gas turbine engine manufacturers now require an operating lifetime of at least 20,000 hours for sensors that are to be fitted to production models. Although conventional sensors have been successfully used at high operating temperatures for short periods of time, it is unlikely that they will ever be able to meet the required operating lifetime because of the inherent weakness of the sensor assembly caused by the different thermal expansion properties of the metal and ceramic components.
A further problem is the way in which the electrode, shield and outer housing are connected to the transmission cable. With conventional sensor designs, the conductors of the transmission cable are directly connected to the electrode, shield and outer housing at a high temperature region (i.e. a part of the sensor that reaches an elevated temperature in use). Many types of transmission cables (in particular those where the conductors are insulated using mineral compounds) cannot be used at high temperatures and often fail after a short period of time. Furthermore, some conventional sensors do not have a hermetic seal between the transmission cable and the rest of the sensor assembly. This can allow moisture to penetrate the sensor assembly and reduce the performance of the sensor.